More and more telecom operators around the world are gradually migrating originally scattered and incompatible networks that adopt different technologies to unified network architecture based on the Internet protocol (IP) or multi-protocol label switching (MPLS) technology. With advantages such as being flexible and simple, topology-independent and connection-oriented, an IP/MPLS technical system is becoming a major service bearing manner adopted by the telecom operators in a network.
The IP/MPLS technical system mainly includes two parts, namely, a control layer and a forwarding layer. The control layer is mainly responsible for discovering an end-to-end route and establishing a path in a network, and the forwarding layer is mainly responsible for addressing and distributing of packets. Main protocols defined at the control layer include the open shortest path first (OSPF) protocol, the intermediate system to intermediate system (IS-IS) protocol, the label distribution protocol (LDP), the resource reservation protocol (RSVP), and so on. Main specifications defined at the forwarding layer include IP, MPLS and so on.
The MPLS uses a short label of a fixed length to an encapsulate network layer packet. Two important concepts in the MPLS are forwarding equivalence class (FEC) and label.
The MPLS is actually a class-based forwarding technology that classifies packets with a same forwarding processing manner (having a same destination, using a same forwarding path, and having a same service level) into one class, which is referred to as an FEC. Generally speaking, classification of a packet into an FEC is in accordance with its network layer destination address. Packets belonging to a same FEC get completely the same processing in an MPLS network.
A label is a short identifier that is of a fixed length and has a local meaning, and is used to identify an FEC. When packets arrive at ingress of the MPLS network, they are classified into different FECs according to a certain rule, and according to a FEC to which a packet belongs, a corresponding label is encapsulated in the packet. Therefore, in the network, the packet may be forwarded according to the label.
LDP is an important protocol at the control layer of the IP/MPLS architecture, and defines a mechanism and method for establishing a point-to-point LSP between two nodes along a shortest path. At present, the following situation may exist in an LSP established on a ring topology according to LDP: if a link fault occurs on the ring topology, packets are discarded, and such a packet loss process does not stop until a completely new LSP is established. The entire packet loss process generally lasts for a few seconds, which is unacceptable for a mobile voice service.